MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between radio frequency (RF) waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. When a substance, such as human tissue, is subjected to the main magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with the main magnetic field. When excited by an RF wave, the spins precess about the main magnetic field at a characteristic Larmor frequency. A signal is emitted by the excited spins and processed to form an image.
A determination of tissue conductivity and permittivity in an MRI exam may be useful for a number of different purposes. For example, the determination of the electrical properties of tissue (conductivity and permittivity) is useful in estimating or simulating local RF power deposition (also known as local specific absorption rate or abbreviated as SAR) during acquisition of MR images. The electrical properties of tissue can also be useful in discriminating between malignant and healthy or benign tissue (e.g., malignant tissue has been shown to have higher permittivity and conductivity than surrounding healthy tissue). In some applications, knowledge of the electrical properties of tissue can be used during therapeutic applications of heat using radio frequency, for example, RF hyperthermia for treatment planning.
Typically, the determination of tissue conductivity and permittivity in MRI is performed using two separate MRI acquisitions, one to map the magnitude of the transmit field, and one allowing the approximation of the phase of the transmit field. This process, involving two separate MRI acquisitions, is typically slow and results in the acquisition of images with low signal to noise ratio (SNR). Moreover, the mapping of the transmit field involves an acquisition that may not exist on all clinical scanners, which has the additional drawback of being SNR inefficient.